In transmissions between communication devices, different types of information may be transmitted. A transmission may comprise e.g. data and control signals as well as Reference Signals, RSs. The control signals, also called control signalling, and the RSs enable the recipient of the transmission to successfully receive and decode, or demodulate, the transmission in order to retrieve the data comprised in the transmission.
In this disclosure, terminology from the 3rd Generation Partnership Project, 3GPP, Long Term Evolution, LTE, advanced will be used to exemplify different embodiments. However, the use of terminology should not be seen as limiting to the scope of the embodiments but as exemplifications of the same. Other wireless systems, e.g. Wideband Code Division Multiple Access, WCDMA, Worldwide Interoperability for Microwave Access, WiMax, Ultra Mobile Broadband, UMB, and Global System for Mobile Communications, GSM, may also benefit from exploiting the ideas covered within this disclosure.
LTE networks are designed with the aim of enabling optional CoMP (Coordinated multipoint processing) techniques, where different sectors and/or cells operate in a coordinated way in terms of, e.g., scheduling and/or processing. An example is uplink, UL, CoMP where the signal originating from a single UE is typically received at multiple reception point and jointly processed in order to improve the link quality. UL joint processing (also referred to as UL CoMP) allows transformation of what is regarded as inter-cell interference in a traditional deployment into useful signal. Therefore, LTE networks taking advantage of UL CoMP may be deployed with smaller cell size compared to traditional deployments in order to fully take advantage of the CoMP gains. Other forms of UL CoMP are however possible, such as coordinated scheduling for cells belonging to the same CoMP coordination area (CoMP cluster). A special case of coordinated scheduling consists of assigning UE specific resources for the UL RS in order to improve interference between co-scheduled UEs, at least on the RS. LTE Release-11 introduces some features for enhanced controlled of UE specific parameters determining the UL Demodulation Reference Signal, DMRS.
The LTE UL is designed assuming coherent processing, i.e., the receiver is assumed to be able to estimate the radio channel from the transmitting user equipment, UE, and to take advantage of such information in the detection phase. Therefore, each transmitting UE sends a reference signal (RS) associated to each UL data or control channel (e.g. Physical Uplink Shared Channel, PUSCH, and Physical Uplink Control Channel, PUCCH). In case of PUSCH, one Demodulation Reference Signal, DMRS, per slot is transmitted on the same bandwidth as the uplink data channel. In case of PUCCH, multiple PUCCH-RSs are transmitted and time multiplexed by the UE within each subframe, spanning the PUCCH bandwidth assigned to the UE.
Additional RS possibly transmitted by UEs comprises Sounding Reference Signals, SRSs, i.e. signals that are transmitted by a UE at predetermined time instances and over a predetermined bandwidth, in order to enable estimation of the UL channel properties at the network side.
RSs from different UEs within the same cell potentially interfere with each other and, assuming synchronized networks, even with RS originated by UEs in neighbouring cells. As already mentioned, in order to limit the level of interference between RSs different techniques have been introduced in different LTE releases in order to allow orthogonal or semi-orthogonal RSs. The most common deployment principle of LTE assumes orthogonal RS within each cell and semi-orthogonal RS among different cells. However, orthogonality of DMRS transmitted by UEs belonging to different cell is supported for 3GPP Release-11 UEs.
Each RS is a pseudo-random signal generated in the frequency domain, enjoying some special properties that make it suitable for channel estimation. The signal corresponding to each RS is determined by a Base Sequence Index, BSI, a CS, and possibly an Orthogonal Cover Code, OCC.
A group-index and a sequence-index define the so called BSI. BSIs are assigned in a UE-specific fashion in Release-11. Different base sequences are semi-orthogonal, which implies that some inter-sequence interference is typically present. The DMRS for a given UE is only transmitted on the same bandwidth of the corresponding data signal (e.g. PUCCH, PUCCH) and the base sequence is correspondingly generated so that the RS signal is a function of the bandwidth. For each subframe, 2 RSs are transmitted, one per slot.
In order to minimise the impact of interference peaks on RSs, interference randomization techniques have been introduced in LTE. In particular, sequence hopping and group hopping (jointly referred to as SGH) are randomisation techniques which operate on a slot level. In case of SRS, SGH operates on a subframe level, since only one SRS symbol per subframe is typically generated (with the exception of certain Uplink Pilot Time Slot, UpPTS, configurations). SGH can be enabled/disabled on a cell-basis by use of the cell-specific parameters Group-hopping-enabled and Sequence-hopping-enabled, affecting respectively group hopping and sequence hopping. For 3GPP Release-10 and later UEs, SGH can be disabled in a UE specific fashion by setting the UE-specific RRC parameter Disable-sequence-group-hopping. Additionally, cyclic shift hopping, CSH, patterns provide further RS interference randomisation by applying a UE-specific pseudo-random cyclic shift, CS, per slot. According to the CSH pattern, a different CS offset may in general be applied in each slot and it is known at both UE and evolved e-Node B, eNB sides, so that it may be compensated at the receiver side during channel estimation.
CSs are linear phase shifts applied to each BSI in the frequency domain. OCCs are orthogonal time domain codes, operating on the RSs provided for each UL subframe (OCC can be in principle applied to an arbitrary number of RSs).
Orthogonal DMRS between UEs may be achieved by use of cyclic shift, CS, if the UEs have the same bandwidth, BW, and BSI, and OCCs if the UEs do not employ sequence group hopping, SGH, and employ the same cyclic shift hopping, CSH, pattern. CS is a method to achieve orthogonality based on cyclic time shifts, under certain propagation conditions, among RS generated from the same base sequence. Only 8 different CS values may be dynamically indexed in Releases-8/9/10, even though in practice less than 8 orthogonal DMRS may be achieved depending on channel propagation properties (without considering OCC in this example). Even though CS is effective in multiplexing DMRSs assigned to fully overlapping bandwidths, orthogonality is lost when the bandwidths differ and/or when the interfering UE employs another base sequence or CSH pattern.
The OCC code [1 −1] is able to suppress an interfering DMRS as long as its contribution after the matched filter at the receiver is identical on both DMRSs of the same subframe. Similarly, the OCC code [1 1] is able to suppress an interfering DMRS as long as its contribution after the eNB matched filter has opposite sign respectively on the two RSs of the same subframe.
While base-sequences are assigned in a semi-static fashion, CS and OCC are dynamically assigned as part of the scheduling grant for each UL PUSCH transmission, for PUSCH DMRS. The CS/OCC assignment method for PUCCH DMRS is different.
Even though different implementations are possible, the typical channel estimator performs a matched filter operation of the received signal corresponding to each RS with the corresponding transmitted RS (the operation can be equivalently performed in time or frequency domains). If OCC is applied, the multiple RSs spanning the OCC code are combined according to the corresponding OCC.
CS-based orthogonality is only effective when the co-scheduled UEs are allocated the same BW, which is a serious scheduling limitation. Therefore, OCC appears as the main tool for achieving inter-UE orthogonality, since OCC is effective when the BW of the co-scheduled UEs is not fully overlapping. SGH needs to be disabled in order for OCC to be effective. Since only 2 OCC codes are available in LTE for, e.g. PUSCH DMRS, a subset of the UEs that are scheduled in the system will be assigned the first OCC code, and a subset of the UEs will be assigned the second OCC code. UEs with the same OCC code produce mutual (inter-UE) interference.
Due to SGH disabling, inter-UE interference for UEs with the same OCC code is on average 3 dB larger than for UEs with SGH enabled. This issue degrades the potential gains for RS coordination.